Just wanted to add on to vulkanino's answer... now 10 years later ha. This is what I was looking for maybe someone else is too. You can also build on multiple =SUBSTITUTE() functions.
Here is what I used to format a list of names for Active Directory accounts. B2 is where the name would go. I wanted to change the space between the First name and last to a period and omit any hyphens from names.
=SUBSTITUTE(SUBSTITUTE(B2, " ", "."), "-", "")
For example the line above would look like this:
First Last-Name => First.LastName
, "-", "")
This part removes the hyphen.
(B2, " ", ".")
This part selects the cell to modify and changes empty space to a period.
=SUBSTITUTE(SUBSTITUTE(...)...)
I used two substitute functions one inside the other.